The present invention relates to a process for the production of polyether polyols which is catalyzed with a Lewis-acid metal perfluoro-alkylsulfonate.
Polyether polyols are obtainable by polyaddition of alkylene oxides (such as ethylene oxide, propylene oxide, or butylene oxide) to compounds having active hydrogen atoms (also known as "starter compounds") such as alcohols, amines, acid amides or phenols. Polyether polyols are used to produce polyurethane plastics, surfactants and lubricants. The polyaddition of epoxides to starter compounds is generally conducted in known manner by alkali catalysis. The alkali catalysts typically used are alkali hydroxides. Among the disadvantages of alkali hydroxide catalyzed polyether polyol production methods are the long reaction-times (&gt;5 hours) and the elaborate product processing necessitated by neutralization of the alkaline polymer. (See, for example, U.S. Pat. Nos. 4,129,718; 4,482,750 and 4,029,879; JP 73 26 391; and Encyclopedia of Polymer Science & Eng, Vol. 6, New York 1986, pages 273-307.) One of the problems encountered in these known processes is the base-catalyzed rearrangement of epoxides (e.g., propylene oxide) which takes place as a side-reaction and yields allyl or propenyl alcohols and monofunctional polyethers having a terminal double bond.
Acid catalysis (particularly with Lewis acids such as boron trifluoride) has also been used for the polyaddition of alkylene oxides to starter compounds. Production of polyether polyols by acid catalysis has the disadvantages of promoting side-reactions (e.g., the formation of volatile, low-molecular weight cyclic ethers) to an increased extent, substitution of hydroxyl groups with acid anions, and broader molecular weight distribution of the polyols than polyols produced by base catalysis. The fact that it is difficult to separate Lewis-acid catalysts from the reaction mixture and the susceptibility of acid catalysts to hydrolysis (necessitating use of special materials (e.g., enamels)) are also disadvantageous. Control of the reaction is also difficult due to the high activity of the catalyst.
U.S. Pat. No. 4,543,430 discloses a process for the production of addition products from an alkylene oxide or epichlorohydrin and hydroxyl group-containing compounds in the presence of trifluoromethanesulfonic acid salts (triflates) of the alkali metals, the metals in the 2nd group of the Periodic Table of Elements and also of the elements aluminum, cobalt, nickel, zirconium and tin. The use of aluminum triflate and zinc triflate is taught to be preferred. However, to achieve a high selectivity, it is necessary to use a large excess of hydroxyl group-containing compound. (See column 2, lines 51-53 of U.S. Pat. No. 4,543,430).
The process described in U.S. Pat. No. 4,543,430 is unsuitable for the production of polyether polyols by polyaddition of alkylene oxides to compounds having active hydrogen atoms (starter compounds) because this disclosed process produces a very high proportion of undesirable by-products (e.g., low-molecular cyclic ethers such as 1,3-dioxolane and 1,4-dioxane) and the polyether polyols obtained are dark brown in color due to the high by-product content. (See Comparative Example 7.)
EP-A 212,820 discloses a process for the production of glycol ethers in which an alkylene oxide is reacted with an alcohol in the presence of an aluminum perfluorosulfonic acid polymer catalyst. A high selectivity of the 1:1 adduct is attained by using a large excess of the alcohol. (See Example 3 in EP-A 212,820.)
With a view to increasing the selectivity, EP 569,331 discloses a process for the production of addition products in which an alcohol is reacted with an epoxide compound in the presence of a complex metal compound of a metal from the main groups or subgroups of the Periodic Table of Elements containing sulfonate residues of a perfluoro alkanesulfonic acid and also at least one weakly bonded, neutral monovalent or polyvalent ligand. Particularly suitable for this process is a metal-complex compound of the formula La(CH.sub.3 CN).sub.x (H.sub.2 O).sub.Y (CF.sub.3 SO.sub.3).sub.3. (See claim 12 in EP-A 569,331.) A disadvantage of these metal-complex catalysts is the need for laborious separation and total recovery of the complex system composed of metal perfluoroalkyl sulfonate and ligand from the polyol reaction mixture. The low catalytic activity of these metal-complex compounds makes it necessary to use large quantities of catalyst in the polyether production process. (See Comparative Example 9.) The production of polyethers with these metal-complex compounds would therefore be very uneconomical.
WO 95/02625, WO 95/02626 and WO 96/13540 describe the polymerization of cyclic ethers (oxiranes, oxetanes, tetrahydrofurans, oxepanes, 1,3-dioxolanes or 1,3,5-trioxanes) to form linear polyethers by catalysis with metal perfluoroalkylsulfonates in the presence of an accelerator (co-catalyst) such as a carboxylic acid anhydride, a carboxylic acid, an acid chloride, a vinyl ether, and certain phosphorus or silicon compounds. Particularly preferred metals are scandium, yttrium and the rare-earth metals ytterbium, dysprosium, erbium, neodymium and lanthanum. (See WO 96/13540, page 6, line 35 and page 7, lines 1-2.) It is recommended that protic compounds (e.g., water and alcohols) be carefully excluded by suitable measures (e.g., drying of the initial materials). (See page 8, lines 23-27 in WO 96/13540.) The process described in these disclosures is therefore not suitable for the production of polyether polyols by polyaddition of epoxides to starter compounds having active hydrogen atoms.